Even though it is in the correct section (Firearm Laws and Regulations) I must admit it took me a while to find it and I knew it existed.
I can see how it could be overlooked by someone who didn't know it was here somewhere.
Maybe I'll add a link in the builders forum to make it easier to find.

if doing a AR you can do multiple calibers.
My SBR which I just got approved, has 22lr/9mm/45acp/.223
I don't think it really matters, but I wanted to cover all bases.

I am confused now. I read a letter from the ATF stating only a single caliber would be approved on each application.....I just did my AR-15 SBR and got it back approved. I wonder if I can ammend the calibers to include the others ?